14-1 Overla of Three adia CHAPTER 14 nd in propene is relatively weak Delocalized Pi Systems: Investigation by Ultraviolet and mawt Visible Spectroscopy 3.The(hat) e学e-e-” H 8o2n (ay) thoth cven ∠88 m二a@w。 af'm“人 1
1 CHAPTER 14 Delocalized Pi Systems: Investigation by Ultraviolet and Visible Spectroscopy Overlap of Three Adjacent p Orbitals: Electron Delocalization in the 2-Propenyl (Allyl) System 14-1 Three key observations: 1. The primary C-H bond in propene is relatively weak. 2. Compared to saturated primary haloalkanes, 3-chloropropene dissociates relatively fast under SN1 conditions and undergoes rapid unimolecular substitution through a carbocation intermediate. The 3-propenyl cation is more stable than other primary carbocations. Its stability has been found to be about equal to that of a secondary carbocation. 3. The pKa of propene is about 40 (that of propane is about 50). The formation of the propenyl anion by deprotonation appears to be unusually favored. Overlap of Three Adjacent p Orbitals: Electron Delocalization in the 2-Propenyl (Allyl) System 14-1 Delocalization stabilizes 2-propenyl (allyl) intermediates. In each of the preceding three unusual observations, a reactive center (a radical, a carbocation or a carbanion, respectively) was formed adjacent to a double bond. This arrangement imparts special stability to the reactive center in question through delocalization of the reactive center: The 2-propenyl (allyl) π system is represented by three molecular orbitals. The molecular orbital description of resonance involves 3 parallel p orbitals, each on an adjacent carbon atom:
Hybridizing 3 atomic p orbitals results in 3 molecuar orbitals: .bonding,no nodes meegab5sapearabasaotbakn 一 888 水 8"g 888 The electron density in the allyl system: 14-2 Radical Allylic Halogenation Thechain mechanism, 一 -cc-cccCHC-c 0 2
2 Hybridizing 3 atomic p orbitals results in 3 molecular π orbitals: 1. π1: bonding, no nodes 2. π2: nonbonding, one node (same energy as p orbitals) 3. π3: antibonding, two nodes The Aufbau Principle is used to fill up the available π orbitals in order of lowest to highest energy. The electron density in the allyl system: 14-2 Radical Allylic Halogenation Under conditions of low halogen concentration, halogens add to allylic molecules through a radical chain mechanism called radical allylic substitution. N-bromobutanimide (N-bromosuccinimide) suspended in chloroform is often used in allylic brominations. It is nearly insoluble and reacts with trace amounts of HBr to generate very small amounts of bromine. The reaction mechanism is through a radical chain mechanism, initiated by light or traces of radical initiators. The resonance-stabilized radical can react at either end of the allylic system to produce an allylic bromide and another bromine radical
14-3 Nucleophilic Substitution of Allylic Halides: SN1 and SN2 Allylic halides can also undergo S2 reactions. GIC-CRC CHcw-cu CH-CHCHA CH-CICH CH.CHCHO CHCHCH C 14-Allylic Organometallic Reagents:Useful 14-5 Two Neighboring Double Bonds:Conjugated Inree-Carbon N rorm to o CH-CH-CH-CH The Grignard derivative can also be prepared: lithium and Grianare s can function as g3 2agh2ltcerg3todandnonrcejugtaddenss Teioe2tostancnorDeratngbetraobenstondan indicate about the c-c-c 3
3 Nucleophilic Substitution of Allylic Halides: SN1 and SN2 14-3 Allylic halides undergo SN1 reactions. Different allylic halides may give identical products if they dissociate into the same allylic cation. If an unsymmetrical allylic radical is formed, a mixture of products may be obtained. Allylic halides can also undergo SN2 reactions. SN2 reactions of allylic halides with good nucleophiles are faster than those of the corresponding saturated haloalkanes due to transition-state stabilization by overlap of the double bond and the p orbital. Allylic Organometallic Reagents: Useful Three-Carbon Nucleophiles 14-4 Alkyl lithium reagents can be made from propene derivatives by proton abstraction by an alkyl lithium. The Grignard derivative can also be prepared: Allylic lithium and Grignard reagents can function as nucleophiles. Two Neighboring Double Bonds: Conjugated Dienes 14-5 Hydrocarbons with two double bonds are named dienes. Dienes can be either non-conjugated (separated by at least one saturated carbon atom) or conjugated (immediately adjacent to one another). In a conjugated system the single atomic p orbital on each outer sp2 hybridized carbon atom overlaps one of the two perpendicular p orbitals on the inner sp hybridized carbon atom. Naming of conjugated and non-conjugated dienes is straightforward. The longest chain incorporating both double bonds is found and numbered to indicate the locations of substituent groups. Cis-trans, or E,Z prefixes indicate the configuration about the double bonds
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4 Conjugated dienes are more stable than nonconjugated dienes. The heat of hydrogenation of a single non-conjugated double bond is about -30 kcal mol-1, regardless of its position within an alkene. In the case of a conjugated diene, such as 1,3-butadiene, however, the heat of hydrogenation is less: This difference in energy is known as the resonance energy of 1,3-butadiene. Conjugation in 1,3-butadiene results from overlap of the π bonds. The conformation of 1,3-butadiene places the p orbitals of C2 and C3 parallel to each other, which permits the formation of a weak π interaction of a few kilocalories per mole. The weak C2-C3 π interaction also raises the barrier to rotation about the single bond to more than 6 kcal mol-1. The s-cis conformation is almost 3 kcal mol-1 less stable than the s-trans conformation. The molecular orbital diagram for butadiene shows two bonding molecular orbitals and two antibonding molecular orbitals. Electrophilic Attack on Conjugated Dienes: Kinetic and Thermodynamic Control 14-6 Even though they are more stable than dienes with isolated double bonds, conjugated dienes are actually more kinetically reactive with electrophiles and other reagents
Manyeleoladditions todiees yleldprodut mxtures: 立人立 o-o-oooo-co- 。t 14-7 Delocalization Among More than Two Bonds: Extended Conjugation and B llee ds are in conjugation,the molecule is -o.- 5
5 The formation of two products can be understood by examining the reaction mechanism: Many electrophilic additions to dienes yield product mixtures: Changing product rations: kinetic and thermodynamic control. When carried out at 40o C rather than at 0o C, the hydrobromination of 1,3-butadiene gives a 15:85 ratio of bromobutenes, rather than a 70:30 ratio: At the higher temperature, the two isomers are in equilibrium with concentrations reflecting their relative thermodynamic stability. Under these conditions, the reaction is said to be under thermodynamic control. The product distribution at 0o C can be understood by looking at the potential energy diagram for the reaction. After the initial protonation, attack by the bromide is faster at C3 because of two factors: •C3 (secondary) carries more positive charge than C1 (primary). •After losing its proton, the bromide ion is closer to C3 than to C1. Delocalization Among More than Two π Bonds: Extended Conjugation and Benzene 14-7 Extended π systems are thermodynamically stable but kinetically reactive. When more than two double bonds are in conjugation, the molecule is called an extended π system. 1,3,5-hexatriene is quite reactive, polymerizes readily in the presence of electrophiles, but is also relatively stable thermodynamically. Electrophilic additions proceed through a highly delocalized carbocation